Cleaning Up on Dirty Coal

Cleaning Up on Dirty Coal

Cheap coal: This demonstration plant in Wilsonville, AL, uses a transport gasifier to turn two tons of cheap, low-quality coal per hour into a clean-burning gas. A plant based on similar technology is scheduled for China.

The industrial boomtown of Dongguan in southeast China’s Pearl River Delta could soon host one of the country’s most sophisticated power plants, one that uses an unconventional coal-gasification technology to make the dirtiest coal behave like clean-burning natural gas. Its developers, Atlanta-based utility Southern Company and Houston-based engineering firm KBR, announced the licensing deal with Dongguan Power and Chemical Company this month.

Dongguan Power plans to implement the gasification scheme at an existing 120-megawatt natural-gas-fired power plant, turning it into an integrated gasification combined cycle (IGCC) plant that uses cheap, moisture-laden lignite coal. The retrofit should be operating in 2011. That will provide its developers with a demonstration to determine whether technology will work in larger IGCC plants and whether it is a process suitable to integrate carbon capture and storage technology, according to John Thompson, director of the Coal Transition Program for the Clean Air Task Force, a nonprofit environmental consulting firm based in Boston. “They want to show that this works,” says Thompson.

Southern and KBR’s gasification design can use dirty coal because, compared to other gasification reactors, it uses a relatively slow, low-temperature process. Conventional gasifiers, such as General Electric’s and Shell’s, rely on temperatures around 1,500 ºC to turn finely ground coal into a combustible mixture of carbon monoxide and hydrogen known as syngas. Unfortunately, such temperatures melt ash and other mineral contaminants in the coal, forming a glassy slag that eventually eats through the ceramic tiles that protect the reactors’ steel walls. Even reactors using high-quality coal have to be taken out of service for installation of new tiles at least every three years. They are thus ill-adapted for lower-quality coals that would produce several times more slag.

Dongguan’s gasifier will sidestep those issues by operating at just 925 ºC to 980 ºC, below the contaminant melting temperature, explains Randall Rush, Southern Company’s general manager for gasification systems. Coal nevertheless gasifies completely at these lower temperatures because it spends twice as long in Southern and KBR’s process.

The technology is an adaptation of the fluidized catalytic cracking employed in refineries since the 1940s, which processes petroleum by “transporting” it around a loop along with solid catalyst particles. In the gasification reactor, the incoming feed of fresh coal is transported with a looping flow of solid coal contaminants, primarily ash. The hot mass drives off most of the coal’s energy content as syngas. The solids left over simply join the flow.

Southern and KBR began designing the technology in 1988 and, with support from the U.S. Department of Energy, started up a demonstration reactor at Wilsonville, AL, in 1996 that can gasify two tons of coal per hour. Four years ago they redesigned it, incorporating what they’d learned at Wilsonville. Rush says the result will be a comparatively reliable and affordable IGCC design. In the absence of slag, a reactor’s ceramic lining should last 10 to 20 years, says Rush.

The technology is attractive to Dongguan Power because it can use coal that is cheaper and less desirable. Presentations by the firm note that a doubling in fuel costs between 2004 and 2006 eliminated the company’s profit margin. And while Dongguan Power initially commissioned a reactor from the Chinese Academy of Sciences’ Institute of Engineering Thermophysics, which built a demonstration-scale transport gasifier last year, the firm has now opted for KBR and Southern’s design.